Communication products such as radio telephones and the like often use differential circuits to enhance signal processing capabilities. Differential circuits are generally less susceptible to external interference than single-ended circuits, and are used for such applications as direct conversion receivers and direct launch transmitters, which are gaining in popularity. However, a major problem hindering the use of differential circuits in direct conversion receivers is that of even order distortion. As an example of even order distortion, if two signals are introduced to a circuit with the two signals being at different frequencies separated by some difference frequency (.DELTA.f), a third undesired signal is generated at the output at a frequency of .DELTA.f. For instance, if the two input signals are at 100 megahertz (MHz) and 101 MHz, then the undesired signal would be at 1 MHz. In the case of a direct conversion receiver, if the undesired signal at 1 MHz falls in the desired output frequency band, it would create an undesired response in the receiver output circuitry.
In a typical direct conversion receiver, a received radio frequency signal is converted to differential signals that are mixed with injection signals to achieve particular phase and amplitude separation characteristics. Ideally, the mixer circuitry is arranged to process the resultant differential signal in a balanced manner. However, when the mixer circuitry is not ideal, i.e., when there are imbalances in the circuitry, secondary nonlinear responses are formed which result in even order distortion.
Much effort has been expended in circuit design in an attempt to reduce even order distortion, such as second order intermodulation distortion. One prior art solution is to use high-quality components and robust manufacturing techniques to ensure balanced differential processing circuits. This approach can add significant cost to a product. Another approach is to incorporate feedback circuitry that detects imbalances and provide for corrective action to eliminate or reduce the effects of the imbalances. One problem with this approach is that imbalances may not be detected until there is an adverse impact on the differential processing circuit. Moreover, errors induced by imbalances may not be detectable at the output of a differential processing circuit.
It is desirable to reduce the even order distortion performance of differential circuits, for such applications as direct conversion receivers. Circuit techniques employed in the art do not adequately address even order distortion present in differential circuits. Accordingly, a new approach is needed to reduce even order distortion.